如何在C中的TCP套接字上构建协议标头和主体

Question

我正在写一个塔卡奇+客户

我试图编写一个简单的数据包发送到服务器，检查了RFC页面 ，发现协议需要在每个数据包中获取此标头：

    +----------------+----------------+----------------+----------------+
    |major  | minor  |                |                |                |
    |version| version|      type      |     seq_no     |   flags        |
    +----------------+----------------+----------------+----------------+
    |                                                                   |
    |                            session_id                             |
    +----------------+----------------+----------------+----------------+
    |                                                                   |
    |                              length                               |
    +----------------+----------------+----------------+----------------+

现在我对此很陌生，我不确定如何构建此标头，是否正在创建这样的缓冲区足够好？：

snprintf(buffer, sizeof(buffer), "%x%x%x%x%x",version,version,type,seq,flags,session_id,length);

然后以相同的方式获取此缓冲区并将其附加到主体并发送：

send(socket,buffer,sizeof(buffer),0);

还是有另一种方式来完成整个数据包的构建？

Answer 1

您所拥有的根本无法产生正确的输出！

以下是可移植的：

#include <stdint.h>
#include <stdlib.h>
#include <string.h>

uint8_t* pack_uint8(uint8_t* dest, uint8_t src) {
   *(dest++) = src;
   return dest;
}

uint8_t* pack_uint32be(uint8_t* dest, uint32_t src) {
   *(dest++) = src >> 24;
   *(dest++) = (src >> 16) & 0xFF;
   *(dest++) = (src >> 8) & 0xFF;
   *(dest++) = src & 0xFF;
   return dest;
}

uint8_t* build_packet(
   uint8_t maj_version,
   uint8_t min_version,
   uint8_t type,
   uint8_t seq,
   uint8_t flags,
   uint32_t session_id,
   uint32_t body_length,
   const uint8_t* body
) {
   uint8_t* packet = malloc(12 + body_length);
   uint8_t* p = packet;
   p = pack_uint8(p, ( maj_version << 4 ) | min_version);
   p = pack_uint8(p, type);
   p = pack_uint8(p, seq);
   p = pack_uint8(p, flags);
   p = pack_uint32be(p, session_id);
   p = pack_uint32be(p, body_length);
   memcpy(p, body, body_length);
   return packet;
}